The present invention relates generally to planarization of a substrate; and more particularly relates to chemical mechanical polishing (CMP) of a semiconductor substrate employing two separate chemical mechanical slurry solutions.
During fabrication of certain semiconductor devices, a metal layer, e.g., tungsten or aluminum may be deposited upon an underlying patterned dielectric layer, e.g., phosphosilicate glass (PSG), borosilicate glass (BSG), borophosphosilicate glass (BPSG) or silicon dioxide (SiO.sub.2), to form interconnects, for example, within a semiconductor. In forming the interconnects, chemical mechanical polishing of the metal layer removes metal material until reaching the dielectric. Ideally, the polished substrate has a flat surface, leaving metal in voids of the patterned dielectric to serve as interconnect plugs. These interconnects should each have a flat upper surface that is flush with an upper plane defined by the top surfaces of the patterned dielectric layer. When a subsequent metal layer is deposited, the deposited metal electrically connects with ends of the plugs. The interconnect plugs, therefore, provide electrical connections between the upper metal to respective semiconductor regions therebelow.
Available CMP procedures for planarizing a metal layer relative to a patterned dielectric for the formation of an interconnect plug often result in undesirable divits, surface unevenness, or salt residues. Assuming a known planarization procedure employing two separate CMP slurry solutions, it is theorized (pursuant this disclosure), that the addition of a second CMP slurry solution to a polishing procedure while first CMP slurry solution is still present, may alter a pH of the polishing procedure. This change in pH level, in-turn, may effect formation of precipitates, causing the slurry solution to gel and/or provide salt formations.
Furthermore, when injecting different slurry solutions onto a polishing pad at different times during polishing of a semiconductor substrate, the pH of the combined solutions is not precisely controlled; rather, the pH level is in a state of fluctuation as the new slurry solution is dispensed onto the pad and mixed with solutions already on the pad. Thus, the resulting CMP process rate and selectivity are not precisely known. For example, the slurries of certain known CMP processes for polishing tungsten have been found to chemically attack the tungsten layer and create undesirable voids within exposed tungsten plugs, or result in the formation of vias during later process stages. Additionally, if there is a loss of oxide selectivity (for the above described example where the oxide layer serves as the patterned dielectric beneath the tungsten), the lack of oxide selectively may result in complete removal of certain oxide portions so as to expose and damage underlying semiconductor devices beneath the oxide layer--resulting in scrapped material.